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fopen mode中a与a+区别

fopen mode中a与a+区别

有二个区别:
1、a 打开一个文本文件,并在文件末尾追加数据,a+打开一个文本文件,允许读,或在文件末尾追加数据
2、"a"模式在添加数据前并不清除EOF标志. 添加数据后 用 MS-DOS 的 TYPE 命令只能显示数据到原来EOF 标志而并不会显示后来添加的.
fopen()的参数,"a+"模式会清除EOF标志, 添加数据后 用 MS-DOS 的 TYPE 命令能显示所有的数据.  "a+" 模式需要以CTRL+Z EOF标志结束的流输入.

相关的E文说明更清楚

The "a" mode does not remove the EOF marker before appending to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data appended to the file. The "a+" mode does remove the EOF marker before appending to the file. After appending, the MS-DOS TYPE command shows all data in the file. The "a+" mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker.

When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for “update”). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.

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